1. Field of the Invention
The present invention relates to semiconductor devices, display devices, liquid crystal display devices, light-emitting devices, manufacturing methods thereof, or driving methods thereof. In particular, the present invention relates to semiconductor devices, display devices, liquid crystal display devices, and light-emitting devices which include circuits including thin film transistors in which oxide semiconductors are used for semiconductor films, manufacturing methods thereof, or driving methods thereof.
2. Description of the Related Art
Currently, thin film transistors (TFTs) in which silicon layers formed using amorphous silicon or the like are used as channel layers are widely used as switching elements in display devices typified by liquid crystal display devices. Although thin film transistors formed using amorphous silicon have low field-effect mobility, they have an advantage that larger glass substrates can be used.
Further, in recent years, attention has been drawn to a technique by which a thin film transistor is formed using a metal oxide having semiconductor properties and such a transistor is used in an electronic device or an optical device. For example, it has been known that some metal oxides such as tungsten oxide, tin oxide, indium oxide, and zinc oxide have semiconductor properties. A thin film transistor in which a transparent semiconductor layer formed using such a metal oxide is used for a channel formation region has been disclosed (Reference 1).